The present invention generally relates to photoplethysmographic measurement systems and, in particular, to a method and system which provides improved DC and low frequency rejection in a detector output signal prior to amplification thereof.
In the field of photoplethysmography, light pulses from different portions of the electromagnetic spectrum are used to noninvasively determine various blood analyte related values in test subjects. Typically, photoplethysmographic measurement systems, such as pulse oximeters, include a probe for releasably attaching to the tip of a patient""s appendage (e.g., finger, earlobe or nasal septum). The probe directs light signals (e.g., red and infrared signals) generated by first and second light sources into the appendage where the probe is attached. Some portions of the light signals are absorbed by the tissue, and the remaining portions of the light signals pass through patient tissue. The intensity of light passing through the tissue is monitored by a photodetector which produces intensity related signals indicative of light absorbency characteristics of the tissue. Because blood analytes to be identified within the patient""s tissue absorb more light at one wavelength than at another wavelength, the intensity of light signals produced by the photodetector may be used to compute an amount of blood analyte (e.g., oxygen saturation of hemoglobin in arterial blood) present in the blood.
Ideally, the intensity related signals produced by the photodetector will accurately represent the amount of light absorbed by the tissue at different wavelengths. However, the intensity related signals, which typically include AC, DC and low frequency signal components, may be distorted by various factors. For example, the DC and low frequency signal components present in the intensity related signal can be photonic energy emitted by the light sources in the xe2x80x9coffxe2x80x9d state when the light sources are both deactivated. The DC and low frequency signal components may also be attributable to other factors such as, for example, ambient light sensed by the photodetector.
Generally, the high frequency AC portions of the detector signals contain the information that is necessary for calculating the blood analyte related values whereas, the DC and low frequency portion of the detector signal may constitute noise or interfere with processing of the detector signal information. For example, because the DC and low frequency portion of the detector signal is typically much larger than the AC portion, accurate representation of the AC portion of the detector signal may be lost during analog-to-digital conversion. Moreover, the DC and low frequency portion contained in the detector signal may prevent sufficient amplification thereof without saturating the detector signal with undesired DC and low frequency signal component.
Some instrument designs have attempted to address problems associated with such DC and low frequency signal components through the use of a capacitor or filter to separate such components from the AC component of interest. However, such approaches have generally had limited ability to remove low frequency and DC components from the detector signals before amplification, thereby possibly saturating the detector signals with undesirable signal components (e.g., ambient light, or photonic energy emitted in the xe2x80x9coffxe2x80x9d state) if the gain of the amplifier is set too high.
One other instrument design, disclosed in U.S. Pat. No. 4,781,195, attempts to address problems associated with such DC signal components by reducing or eliminating a dark current signal produced by a detector during xe2x80x9cdarkxe2x80x9d intervals when light sources are disabled. This instrument design employs a switch coupled to a timing device to interrupt, during a selected portion of the signal cycle, the flow of electricity from a front end amplifier to a feedback loop that provides dark current correction. However, this switch incorporated into the instrument may reduce the effectiveness of the feedback loop, and the incorporation of the switch limits the amount of gain that can be achieved without saturation.
Thus, there is a need for a photoplethysmographic measurement system which improves the quality of an analog output signal produced by a detector. In particular, there is a need for a system that is capable of removing undesirable signal components (e.g., DC and low frequency signal components) from the detector signal, over a signal waveform cycle, before amplification such that the fall signal cycle can be amplified as desired without saturation.
The present invention is directed to a system and corresponding method for use in a pulse oximeter to improve the way in which DC and low frequency signal components (e.g., photonic energy emitted by the sources in the xe2x80x9coffxe2x80x9d state and/or ambient light) are removed from analog signals produced by a detector. According to the present invention, DC and low frequency signal components are continuously removed from the detector signals during the xe2x80x9cdarkxe2x80x9d intervals when light sources are deactivated and also during xe2x80x9clightxe2x80x9d intervals when one of the light sources is activated. Because the DC and low frequency components are continuously canceled from the detector signals prior to amplification thereof, the gain of the amplifier may be increased without saturating the detector signal with undesirable signal components (e.g., noise including ambient light).
In accordance with one aspect of the present invention, photonic energy emitted by the light sources in the xe2x80x9coffxe2x80x9d state and noise (e.g., ambient light) are removed from the detector signals before being amplified. It has been recognized that the light sources may transmit as much as half and sometimes more than half of their photonic power in the xe2x80x9coffxe2x80x9d state. A desirable amount of gain may not be achieved without saturating the detector signals with undesirable signal components if such photonic energy and ambient light are not accounted for prior to amplification of the dectector signal. Because this photonic energy and ambient light are emitted in the form of DC and low frequency signals, the removal thereof from the detector output signals may be accomplished in accordance with the present invention by stripping DC and low frequency signal components from the detector signal over a full cycle of the signal.
In accordance with another aspect of the present invention, a DC restoration circuit is utilized to remove DC and low frequency signal components from the analog signals produced by the detector. The DC restoration circuit includes an amplifier for amplifying the output signals produced by the detector, which may be in the form of an electrical current signal. The DC restoration circuit preferably includes an integrator feedback stage connected to the amplifier to provide an integrator feedback current signal (i.e., opposite in sign to the detector current signal) to cancel undesirable signal components (e.g., DC and low frequency signal components) in the current signals produced by the detector at the input of the amplifier. Because the feedback stage is used to subtract DC and low frequency signal components from the detector signal at the input of the amplifier, a relatively high amplification gain can be achieved without saturating the amplified detector signal with undesirable signal components.
An electrical component (e.g., resistor), may be coupled to the integrator to selectively reduce the gain of the integrator. According to the present invention, any suitable type of integrator capable of providing a high gain at DC and low frequency may be employed. It should be appreciated that the integrator can be specifically configured to define the low frequency xe2x80x9croll-offxe2x80x9d point of the integrator at any desirable frequency level selected to optimize the filtering of the undesirable (low frequency) signal components.
In accordance with a further aspect of the present invention, the configuration of the DC restoration circuit allows for incorporation thereof into pulse oximeters employing a variety of signal multiplexing mechanisms, e.g., including both time multiplexed and non-time multiplexed oximeters. Because the DC restoration circuit of the present invention functions independently of a demultiplexer or a timing device (e.g., light source driver), amplified output signals produced by the DC restoration circuit may be processed by any suitable demultiplexer (e.g., time or frequency division demultiplexer) to analyze the amplified output signal with respect to different wavelengths of light. In one embodiment, the separation of the amplified output signal into different wavelengths is accomplished by a software application running in a processor.
In accordance with yet another aspect of the present invention, a fixed voltage (e.g., zero volts) is maintained across the detector by the amplifier at all times. Because the integrator feedback current signal produced by the integrator feedback stage offsets the DC detector current to maintain a fixed voltage across the detector, a relatively wide range of amplification gain can be achieved without saturating the amplified signal with undesirable DC detector current.